Among the candidate genetic loci for alcoholism and for differences in alcohol's toxic effects are genes encoding alcohol metabolic enzymes. Class III ADH (ADH5) is an unusual ADH which has an important role in the metabolism of formaldehyde (Koivwasalo, 1990). This enzyme could thus play a role in methanol toxicity and in the metabolism of methanol found as a trace contaminant in alcoholic beverages. We have shown that this is the only ADH present in significant amounts in brain. We cloned the human class III alcohol dehydrogenase and determined that it was highly distinct in sequence as compared to the previously known ADHs. The three ADH classes were approximately equal in their sequence identities to each other. Analysis of tissue mRNA levels revealed that class III ADH, unlike the other ADHs, was expressed in all tissues. A moderately informative human Sac I RFLP was discovered. We located the gene to the same chromosomal region in mouse and man, approximately 5 cM from the other ADH genes.